Semiconductor laser with a superbroadband or multiline spectral output

ABSTRACT

A superbroadband or multiwavelength laser transmitter source for wavelength-division multiplexing, two-wavelength interferometry, differential lidar and optical storage applications. Contrary to conventional tunable laser sources that can switch between different lasing wavelengths in a given wavelength band, the superbroadband laser simultaneously emits at multiple wavelengths. The basic idea of this system is to maintain simultaneous lasing operation in an optical active gain medium at different wavelengths. The system uses a novel dispersive cavity. By designing this cavity structure appropriately, the system creates its own microcavities each lasing at a different wavelength within the fluorescence band of the gain medium. Mode competition in the proposed cavity is absent and spectral range of simultaneous multi-frequency generation is considerably enhanced practically to the spectral width of the active media luminescence spectrum. As a result, the radiation of each mode with its own wavelength is amplified in the active media independently from the simultaneous amplification of the rest of the wavelengths.

REFERENCE TO RELATED APPLICATION

This application corresponds to U.S. provisional application Ser. No.60/017,443 filed May 17, 1996 and claims priority thereof, and thisapplication expressly incorporates said provisional application Ser. No.60/017,443 herein by reference.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates to the field of Quantum Electronics, andmore particularly to the lasers that can be widely used as a powerfultool for solving problems in optical telecommunication, informationcoding, interferometry, optical storage, and high resolution, lowcoherent and time resolved imaging spectroscopy.

Primarily, the invention can be used in cases when polychromatic laseremission with a superbroadband continuum or multiline pre-assignedspectral output is required.

II. Description of the Prior Art

The ability to generate and modulate multiple wavelengths is useful formany applications in science and engineering, e.g., in differentialspectroscopy and lidar and in spectroscopic and holographicimplementations of optical data storage. Recent efforts in this fieldhas focused on design of multiple wavelength transmitters forwavelength-division multiplexed (WDM) communications systems. Inparticular, the multichannel grating cavity (MCG) laser or themultistripe array grating integrated cavity (MAGIC) laser have beendesigned and developed. In this lasers several array ridges were pumpedseparately to generate output at different wavelengths. However,compensation for optical cross talk can be a significant design issuefor these systems as well as a lack of flexibility in the selection ofwavelengths or wavelength spacing. For either schemes elaboratetechnology of fabrication and packaging for multistripe diode array arerequired.

This invention relates to an alternative design using a commercial laserdiode or diode array with an external cavity. By designing this cavitystructure appropriately, the system creates its own microcavities eachlasing at a different wavelength within the fluorescence band of thegain medium. Mode competition in the proposed cavity is absent andspectral range of simultaneous multi-frequency generation isconsiderably enhanced practically to the spectral width of the activemedia luminescence spectrum. As a result, the radiation of each modewith its own wavelength is amplified in the active media independentlyfrom the simultaneous amplification of the rest of the wavelengths. Theproposed laser transmitter is suitable for implementation at any of themajor spectral bands (0.8, 1.3, and 1.5 μm) and is anticipated to beoperated in a single longitudinal mode.

The proposed system is based on the principles of superbroadbandoscillation—realization of simultaneous lasing in the whole spectralregion of active medium amplification band. Usually competition ofamplification in laser active medium restricts the spectral range ofsimultaneous coexistence of different wavelengths of lasing. To solvethis problem it is necessary to realize independent oscillations of thecertain parts of the active medium with the corresponding wavelengths.

Several authors have shown the possibility of superbroadband oscillationin the pulsed dye laser, exhibiting, unfortunately, the existence of thesecondary parasitic modes in the resonator, thereby decreasing the rangeof a broadband oscillation. In our recent papers the first solid statesuperbroadband and multi-frequency laser was proposed on the basis ofLiF color center crystals and lasing was realized in practically thewhole amplification spectral region of LiF:F₂ ⁻ crystals.

The prior art is represented by the following:

I. H. White, “A multichannel grating Cavity Laser for WavelengthDivision Multiplexing Applications”, IEEE J. Lightwave Technology 9,893-899 (1991).

K. R. Poguntke and J. B. D. Soole, “Design of a Multistripe ArrayGrating Integrated Cavity (MAGIC) Laser”, IEEE J. Lightwave Technology,11, 2191-2200 (1993).

M. B. Danailov and I. P. Christov, “Amplification of Spatially dispersedultrabroadband laser pulses”, Opt. Commun., 77, 397-401 (1990).

M. B. Danailov and I. P. Christov, “Ultrabroadband Laser UsingPrizm-Based “Spatially-Dispersive” Resonator”, Appl., Phys. B, 51,300-302 (1990).

T. T. Basiev, S. B. Mirov, Room Temperature Tunable Color Center Lasers,Laser Science and Technology books series vol. 16 pp. 1-160. Gordon andBreach Science Publishers/Harwood Academic Publishers, 1994.

T. T. Basiev, S. B. Mirov, P. G. Zverev, I. V. Kuznetsov, R. Sh.Teedeev, “Solid State Laser with Superbroadband or Control GenerationSpectrum” Ser. No. 08/042,217; filed Apr. 2, 1993, US patent pending.

T. T. Basiev, P. G. Zverev, S. B. Mirov, “Superbroad-Band Laser on LiFColor Center Crystal for Near—Infrared and Visible Spectral Regions”,Abstr. Rep. International Conf. “LASER-93”, Munich, Germany, 1993.

T. T. Basiev, P. G. Zverev, S. B. Mirov, V. F. Federov, “Solid StateLaser with Superbroadband or Control Generation Spectrum” SPIE, vol.2379, 54-61, 1995.

T. T. Basiev, P. G. Zverev, V. V. Fedorov, S. B. Mirov, “Multline,superbroadband and sun-color oscillation of a LiF:F₂ ⁻ color-centerlaser”, Applied Optics 36, 2515-2522 (1997).

SUMMARY OF THE INVENTION

The object of the present invention is to provide a semiconductor lasertransmitter which is capable of lasing in multiple wavelength orsuperbroadband regimes of operation.

These and other objects are achieved by application of a novel externalcavity for individual diode or laser diode array, wherein system createsits own microcavities each lasing at a different wavelength within thefluorescence band of the semiconductor gain medium.

According to this invention, the emission from the whole pumped volumeof the diode passes through the focusing means into the aperture, whichseparates from the amplified emission only a part of it, that is spreadparallel to the resonator axis and suppresses all the off-axis modes ofradiation. The transmitted emission is retroreflected by the grating andis repeatedly directed through the gain medium through the indicatedpaths. As a result the radiation of every assigned wavelength isamplified without any mode competition and independently. The outputradiation of the diode consists of the continuous number of beams whichpass in the semiconductor crystal parallel to each other and to thelaser cavity optical axis. Each of them has a certain angle incident todiffraction grating and, consequently, a distinct oscillating wavelengthwhich is determined by a standard equation for diffraction gratingworking in the autocollimation regime: λ=2t sinθ, where “t” is thegrating spacing and “θ” is the incident angle. There is no interactionbetween these beams and it is possible to state that each part of thecrystal parallel to the laser axis works as an independent laser withits own oscillating wavelength.

BRIEF DESCRIPTION OF THE DRAWINGS

The herein described advantages and features of the present invention,as well as others which will become apparent, are attained and can beunderstood in more detail by reference to the following description andappended drawings, which form a part of this specification.

It is to be noted, however, that the appended drawings illustrate onlyexemplary embodiments of the invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block-scheme of an embodiment of a semiconductor laser witha superbroadband or multiline spectral output.

FIG. 2 illustrates an example of particular external cavity design witha spherical focusing element.

FIG. 3 illustrates another example of particular external cavity designwith a “selfock” (gradient index imaging lens) intracavity microopticalfocusing element: (a) a top view of the system; (b) a side view of thesystem.

FIG. 4 illustrates the superbroadband spectral output of the LiF:F₂ ⁻laser with a similar to the above mentioned external cavity design.

FIG. 5 illustrates a fragment of the LiF:F₂ ⁻ color center lasermultifrequency spectral output formed due to spatial filtering element,having a periodic structure and installed into the pumping beam.

DETAILED DESCRIPTION OF THE PEFERRED EMBODIMENTS

FIG. 1 is a block-scheme of an embodiment of a semiconductor laser 100with a superbroadband or multiline spectral output. Superbroadband ormultiline semiconductor laser 100 includes the following components:electrically or laser pumped diode or diode array 102 operating at anyof the major spectral bands (0.8, 1.3, and 1.5 μm), multichanneldispersion and collimation optics 104, and output 106 and rear couplers108.

Emission from the whole pumped volume of the diode 102 passes throughthe multichannel dispersion and collimation optics 104 that directs thespontaneous emission of the diode into the off-axis mode suppressionelement, which separates from the amplified emission only part of it,that is spread parallel to the resonator axis. This separated radiationis diffracted on the dispersive element (which is a part of themultichannel dispersion and collimation optics means as well as theoff-axis mode suppression element) and then is retroreflected by therear coupler 108 back to multichannel dispersion and collimation opticssystem. The off-axis mode suppression element, of the system againextracts from the diffracted radiation only the radiation of the mainlaser modes and secondary laser modes, which diverge from the opticalaxes are expelled from the process of generation. The radiations of themain laser modes, each with a distinct wavelength, are directed back tothe active medium. Each mode with its different wavelength has its owntrajectory. This radiation along with the stimulated radiation provokedby it are retroreflected by the output coupler 106. This process givesrise to the superbroadband multi-frequency oscillation—independentoscillation of the different parts of active medium with differentwavelengths (covering practically the whole spectral region of theactive medium amplification band) and the same output direction. Themultichannel dispersion and collimation optics means may additionallycontain a spatial filtering element (periodic mask or acousto-opticfilter) that will provide an opportunity of rapid switching of thespectral profile of the beam from superbroadband continuum to apre-assigned spectral composition.

The modulation of output radiation can be provided independently foreach spectral channel by the methods of intra- or out of-cavity acousto-and electro-optic modulation, or by the methods of high-frequency pumpcurrent modulation for each laser diode in a bar or array.

Referring to FIG. 2 that illustrates an example of particular externalcavity design with a spherical focusing element the laser operates asfollows:

Active medium, which is a single diode or diode array is electricallypumped or pumped by the radiation of another laser. The diode has areflectivity coating (HR) on the rear facet and an antireflection (AR)coating reflectivity on the front facet. Emission from the whole pumpedvolume of the active medium passes through the focusing element (L) intothe off-axis mode suppression element (A), which separates from theamplified emission only part of it, that is spread parallel to theresonator axis. This separated radiation is diffracted on the grating(G). The grating works as an output coupler in the auto-collimatingregime in the first order of diffraction and retroreflects radiation tothe suppression element (A). The off-axis mode suppression element, inturn extracts from the diffracted radiation only the radiation of themain laser modes and secondary laser modes, which diverge from theoptical axes, and are expelled from the process of generation. Theradiation of the main laser modes, each with a distinct wavelength, iscollimated by the focusing element (L) and directed back to the activemedium. Each mode with its different wavelength has its own trajectory.This radiation along with the stimulated radiation provoked by it areretroreflected by the mirror (HR) on the rear facet of the diode. Thisprocess gives rise to the superbroadband multi-frequencyoscillation—independent oscillation of the different parts of activemedium with different wavelengths (covering practically the wholespectral region of the active medium amplification band) and the sameoutput direction. The output radiation of the superbroadband diode laserconsists of the continuous number of beams which pass in the active zoneof semiconductor crystal parallel to each other and to the laser cavityoptical axis. Each of them has a certain angle incident to diffractiongrating and, consequently, a distinct oscillating wavelength which isdetermined by a standard equation for diffraction grating working in theautocollimation regime: λ=2t sinθ, where “t” is the grating spacing and“θ” is the incident angle (on FIG. 2 θ=α+β). There is no interactionbetween these beams and it is possible to state that each part of thecrystal parallel to the laser axis works as an independent laser withits own oscillating wavelength. The diffraction grating works as anoutput coupler, so that the zero order of diffraction is extracted outof the cavity. The overall output radiation has a spatially spread“rainbow” spectra divergent in the grating dispersion plane, so thateach narrow line laser mode has its own angular direction in the outputradiation. The output radiation is reflected by the mirror (M) (forminga corner reflector (CR) with the grating) to data input and signalmodulation system (DISM) and then focused by the focusing system L₂ intothe fiber. A mask (filtering mask “F”) or image controller installationinto the cavity will result in the multifrequency laser oscillationoutput with the special wavelength distribution or spectral coding.

It is another object of the present invention to provide a semiconductorsuperbroadband and/or multiline laser with various configurations of theexternal cavities based on the micro-optic focusing (cylindrical,spherical, “selfok”—gradient index imaging lenses) and dispersionelements using effects of interference, refraction and diffraction willbe used as well. An example of the particular external cavity design forthe superbroadband semiconductor with a “selfock” (graded-index lens)intracavity micro-optical focusing element is demonstrated in FIG. 3.

The problem is to collimate the radiation in the vertical plane,featuring the dimension of the laser element of about 1 μm and themaximum divergence of about 50°, and to create some dozens of parallel,spectrally and spatially independent channels in the horizontal planewith much bigger dimension “L” of the active zone of the semiconductorcrystal. The length L for a single diode laser element is about 5-100 μmand for a laser bar or array—2×10²-10 ⁴ μm.

It is further object of the present invention to provide the opticalschemes of the superbroadband multi-frequency semiconductor laserscomprising planar waveguide and distributed feedback optical elements.

It is still a further object of the invention to provide the optimalconditions for refraction, interference and diffraction at opticalelements of external cavity and active element to achieve the highestnumber of parallel spectral channels with the minimum spectral width ofeach of them. Besides, the intracavity losses must be decreased to aminimum as well as the interaction (crosstalk) of different spectralchannels should be suppressed.

It is an additional further object of the present invention to provide asuperbroadband semiconductor laser with a further extension of thespectral output by means of arranging of gradients of (a) composition,(b) impurity concentration in the semiconductor material, (c)temperature, and (d) pressure directed along the linear dimension of theemitting diode.

SPECTRAL RESOLUTION OF SUPERBROADBAND LASER IN A MULTILINE OUTPUT MODE

Let's consider that each beam passing through the active zone of thediode is located at a coordinate “y” in the transversal direction, while“y_(o)” corresponds to the position of the optical axis of the laser,“y_(o)” is the angle of intersection of the current beam “y” and theoptical axis and “f” is the focal length of the focusing element L₂.Then we can write: $\begin{matrix}{\alpha = {\arctan \quad {\frac{y_{0} - y}{f}.}}} & (1)\end{matrix}$

If the normal to diffraction grating (G) forms the angle “β” to theoptical axis then one can say that each beam “y” arrives at thediffraction grating at the certain angle determined as “β30 α. Thediffraction grating works in the auto-collimation scheme causing thefirst order of diffraction to be reflected back into the cavity. Sinceeach beam strikes the diffraction grating at a different angle thecavity for each beam has high selectivity for the certain wavelength,determined as

kλ=2t sin (β+α)   (2)

where “k” is the order of reflection (in our particular case k=1), and“t” is the grating spacing. The zero order of diffraction is used as anoutput of the laser.

The spectral resolution of the laser, that is the possibility ofoscillating a certain separate beam with its own wavelength withoutinterfering with another, is determined by the spectral resolution ofthe cavity. This can be estimated by the angular selectivity of thescheme, determined as h/f, where “h” is the size of the aperture. Thismeans that: $\begin{matrix}{{{\Delta \quad \alpha} = \frac{h}{f}},} & (3)\end{matrix}$

and taking into account the diffraction grating equation (2) this gives$\begin{matrix}{{\Delta\lambda} = {2{t \cdot \frac{h}{f} \cdot \cos}\quad {( {\beta + \alpha} ).}}} & (4)\end{matrix}$

So the spectral resolution is determined by the size of the intracavityaperture “h”.

In order to obtain high laser conversion efficiency and high spectralresolution it is necessary to choose the size of the aperture thatallows one to work in a single transversal mode regime but with lowdiffraction losses. This means that one needs to make the Fresnel numberfor each laser ≈1, that is: $\begin{matrix}{{F = {\frac{h^{2}}{L \cdot \lambda} \approx I}},} & (5)\end{matrix}$

where “L” is the length of the cavity. In this case the diffractionlosses of the scheme are not so large, while the spectral resolution isstill high.

Substituting the data of the diode laser in equation (5)—the length ofthe resonator L=10 cm, the focal length f=4 cm of the intracavityfocusing element L, the grating spacing t=1/1200 mm=833 nm, and themaximum of the diode amplification band λ=1.55 μm—one can obtain for thesingle transversal mode oscillation that the size “h” of the aperture“A” must be less than 0.39 mm. The spectral resolution in this case(when the angle of incidence β+α^(˜)85°) will be about 1.4 nm. Furtherreduction of the size of the aperture results in increased spectralresolution and a corresponding growth of the diffraction losses. A lowerlimit to the useful width of the slit is about 20 μm. The narrowestlinewidths in this situation will be about 0.07 nm.

In other words, our laser can combine simultaneously approximately35-700 independent conventional narrowband lasers (or any smallerquantity of lasers, for example 1-10, forming any pre-assigned spectralcomposition), generating synchronously (without any rotation of thegrating) narrowband oscillation with different frequencies, coveringcontinuously the whole spectral range of the diode emission band. Thequantity of these simultaneously generated spectrally narrow lines canbe approximately defined as the width of the active medium fluorescenceband (usually 50 nm for diodes emitting at 1.55 μm) over the single linewidth (0.07-1.4 nm).

EXPERIMENTAL RESULTS

We believe that the superbroadband or multifrequency color center laseris a good object for the multifrequency semiconductor laser modeling.So, we demonstrate a feasibility of the proposed invention on the basisof the superbroadband and multiline LiF:F₂ ⁻ color center laser⁵⁻⁹. Thereason why we believe that the superbroadband or multifrequency colorcenter laser is a good object for the multifrequency semiconductor lasermodeling is as follows:

First of all, color center crystals feature similarity withsemiconductor active media in the value of the gain per pass. Colorcenter crystals have an extremely high for solid state media gain perpass (in cm scale), which is of the same order of magnitude as the gainper pass of the semiconductor lasers (in mm scale).

Secondly, color center crystals demonstrate a very wide emission bands.They are ten times wider than the emission bands of semiconductor activemedia, but the ratio of the spectral widths in the superbroadbandmulti-frequency and single line regimes of oscillations (quantity ofindependent lasing channels) could be of the same order of magnitude,too.

Finally, a planar geometry of optical pumping proposed and realized forthe color center laser provides a good similarity to traditional inplane emitting diode lasers.

The wide beam of the pump neodymium laser longitudinally excites colorcenters through the input dichroic mirror in the large part of the LiFactive element. For pumping, the radiation of the first harmonic ofnanosecond YAG:Nd laser with smooth spatial intensity profile was used.Pulse energy was about 25 mJ. Superbroadband lasing of F₂ ⁻ CCs at thespectral range 1.1-1.24 μm with the efficiency 10-15% was realized in ascheme similar to presented in FIG. 2. FIG. 4 shows one of thesuperbroadband spectra obtained in this experiment. Laser spectrum ofeach pulse was measured with a polichromator and an optical multichannelanalyzer. The changes of laser energy versus wavelength are connectedwith the spatial distribution of the pump laser beam. A special mask(filtering mask “F”) or image controller installation into the pumpbeam, or in the cavity results in the multifrequency laser oscillationoutput with the special wavelength distribution or spectral coding (FIG.5). FIG. 5 representing multi-frequency LiF:F₂ ⁻ laser generation withspectral coding, demonstrate the possibility of rapid switching of thespectral profile of the beam from superbroadband continuum to apre-assigned spectral composition.

These results indicate that there is a significant potential forapplication this new type of superbroadband or control multi-frequencyspectrum generation laser scheme for signal multiplexing or informationcoding.

What is claimed is:
 1. A diode laser or array having a superbroadband orsimultaneous multiple wavelength spectral output when pumpedelectrically, optically or by means of electron beam, the diode laser orarray comprising semiconductor medium and including different gainregions each associated with a spectral component of a desiredwavelength and producing emission radiation which may include secondaryoff-axis laser modes; dispersion means for dispersing the emissionradiation and causing multiple passes of the spectral components throughthe associated gain regions of said active medium and providing anoutput of desired spectral components; and suppression means forsuppressing off-axis laser modes to avoid mode competition therebymaximizing the amplification of the desired spectral components. 2.Laser, according to claim 1, wherein said dispersive means comprises afocusing element and a diffraction grating.
 3. Laser, according to claim2, wherein said diffraction grating forms part of a corner reflectorwhich includes a high reflecting mirror.
 4. Laser, according to claim 2,wherein said focusing element and diffraction grating are monolithicallyintegrated in a planar waveguide structure.
 5. Laser, according to claim2, wherein said diffractions grating is a distributed feedback grating.6. Laser, according to claim 2, wherein said focusing element comprisesa convex lens.
 7. Laser, according to claim 2, wherein said focusingelement comprises graded-index (GRIN) lens.
 8. Laser, according to claim2, wherein said focusing element comprises a cylindrical lens.
 9. Laser,according to claim 2, wherein said dispersion means includes a focusingelement having a focal point at which emission radiation is focused andsaid suppression means comprises an optical blocking element which atleast partially blocks the emission radiation to thereby suppress thesecondary off-axis laser modes.
 10. Laser, according to claim 9, whereinsaid optical blocking element comprises a slit aperture.
 11. Laser,according to claim 1, wherein said active medium comprises asemiconductor crystal emitting in one of the major spectral bands 0.8,1.3 and 1.5 μm.
 12. Laser, according to claim 1, further comprisingfiltering means for limiting the spectral components that aretransmitted through the active medium to those components having desiredwavelengths.
 13. Laser, according to claim 12, wherein said filteringmeans is positioned between said active medium and said dispersionmeans.
 14. Laser, according to claim 12, wherein said filtering meansare fabricated photolitographically as a multistripe mask on thesemiconductor structure.
 15. Laser, according to claim 1, wherein forfurther extension of the range of superbroadband or multiwavelengthspectral output semiconductor medium has a gradient of composition alongthe direction perpendicular to the linear dimension of the emittingdiode.
 16. Laser, according to claim 1, wherein for further extension ofthe range of superbroadband or multiwavelength spectral outputsemiconductor medium has a gradient of impurity concentration along thedirection perpendicular to the linear dimension of the emitting diode.17. Laser, according to claim 1, wherein for further extension of therange of superbroadband or multiwavelength spectral output semiconductormedium has a gradient of temperature along the direction perpendicularto the linear dimension of the emitting diode.
 18. Laser, according toclaim 1, wherein for further extension of the of the range ofsuperbroadband or multiwavelength spectral output semiconductor mediumhas a gradient of pressure along the direction perpendicular to thelinear dimension of the emitting diode.